1. Field of the Invention
This invention relates to filters or traps for removing particulate matter and particularly to such filters employed with engines, such as diesel engines, for removing particulate matter from the exhaust gases thereof.
2. Background of the Invention
It is desirable to remove particulate matter from the exhaust gases of engines in order to minimize the amount of such particulate matter discharged into the atmosphere. It is particularly desirable to provide means for effectively removing particulate matter from the exhaust gases of diesel engines.
Filters of various materials, including filters employing ceramic material, have been employed in the exhaust systems of engines, such as diesel engines, to remove particulate matter therefrom. Normally such prior art systems incorporate a filter of ceramic material and provide for flow of exhaust gases longitudinally through the filter from one end to the other. The length of ceramic filter which can be employed in such systems is limited because of the thermal gradient which occurs over the length of the filter and the resultant thermal stresses induced in the ceramic material.
Such prior art filters are also subject to substantial thermal gradient transversely of the filter, from the center toward the circumference thereof, because the exhaust gases may be for example at a temperature of 600.degree. F., whereas the outer circumference of the ceramic filter may, in extreme conditions, be encountering temperatures as low as -40.degree. F. This temperature gradient also results in substantial thermal stress with the potential of resultant damage to the ceramic filter.
A second deficiency of such prior art filters is that there is substantial heat loss from the filter to the atmosphere, reducing the effectiveness of the filter in burning the particulate matter flowing therethrough because of the reduced temperature of the filter. This heat loss sometimes requires the provision of supplemental heat to effectively dispose of the particulate matter.
Further, prior art particulate filters have the additional problem that in order to keep the back pressure within acceptable limits and still provide adequate surface for effective burning of the particulate matter, it may be necessary to increase the diameter of the filter to an unacceptable degree, that is, the filter diameter may have to be such that a problem is created in achieving an even distribution of exhaust gases across the filter diameter and installing the filter within the space available.
Finally, the conventional filter contributes to inflexibility in design because it materially limits the freedom of the designer to change the aspect ratio, that is the ratio of the length of the filter to its diameter, so that the designer is unable to maximize the filter area while still minimizing thermal stresses.
By the present invention these deficiencies of prior art particulate filters have been eliminated and a filter has been provided which has substantial advantages both in design and in effective operation for its desired purpose.
One object of the present invention is to provide a particulate filter which permits optimization of the aspect ratio, that is, one which permits maximization of the length of the filter relative to its diameter while still minimizing thermal stresses. This also permits limiting the pressure drop through the filter to an acceptable amount while still providing a sufficiently long filter path for effective removal of particulate matter.
Another object of the invention is to provide a more uniform distribution of exhaust gases over the surface of the particulate filter employed so as to reduce the temperature gradient across the filter and also to increase the efficiency of the filter.
Another object of the invention is to provide a particulate filter in which the heat losses from the filter are minimized so that particulate matter is burned more efficiently in passing through the filter and the need for additional heat from an external burner is minimized.
It is still another object of this invention to provide a particulate filter which, while accomplishing the above improved results, is of such shape and size that it can be readily installed in lieu of a conventional particulate filter without the necessity of significant change in the exhaust system.
Finally, it is an object of this invention to provide a particulate filter constructed in such a manner that it acts also as a noise muffler.